In digital radio communication, adaptive array antenna (hereinafter, referred to as “AAA”) technology which adaptively controls the directivity of a plurality of antenna elements by adding weight to the antenna output is used. In the AAA technology, it is possible to suppress the interfering waves by adaptively controlling the directivity utilizing the fact that the directions of arrival of signals are different from each other. Accordingly, the AAA technology is preferred as a method for suppressing the interfering waves.
Referring to FIG. 1, the formation of directivity pattern in a radio base station apparatus which utilizes the AAA technology will be described. FIG. 1 is a diagram showing a transmitted radio wave which is transmitted from a communication terminal apparatus arrives at a radio base station apparatus provided by two antenna elements.
The radio wave transmitted from communication terminal apparatus is received by radio base station apparatus via its respective antenna elements 31 and 32. In this case, the received radio wave 34 which is received by the antenna element 32 introduces a larger phase rotation compared with the received radio wave 33 which is received by the antenna element 31, since the former travels through path difference L longer than that of the latter.
The radio base station apparatus monitors and measures the difference between the phase rotation of the received radio wave 33 and the phase rotation of the received radio wave 34 (difference in phase rotation). Since the difference in phase rotation and the direction of arrival of the received radio wave always related to each other when the fading correlation is high, the radio base station apparatus estimates the direction of arrival of the received radio wave based on the measured difference in phase rotation, and generates a directivity pattern for suppressing the multi-path and other users interference.
However, when the fading correlation between the received radio wave 33 and the received radio wave 34 is small and therefore the fading itself to which the received radio wave 33 and the received radio wave 34 are subjected to in the respective paths is different, the difference in phase rotation measured by the radio base station apparatus includes not only the difference due to the difference in the travelling paths but also the difference due to the difference of the fading itself. Consequently, when the fading correlation is small, the difference in phase rotation measured by the radio base station apparatus and the direction of arrival of the received radio wave do not always related to each other. Thus, since the direction of arrival (direction of angle . in FIG. 1) of the received radio wave can not be estimated correctly when the fading correlation is small, there exists a problem that desired directivity can not be correctly generated leading to deterioration of the communication quality.